This invention relates to an acoustic treatment suppression panel and, more particularly, to an acoustic treatment suppression panel which permits the use of bulk absorbers in an aircraft gas turbine engine environment.
Bulk absorbers have been widely recognized as being acoustically superior to other types of acoustic absorbers in certain applications, such as in the inlets of gas turbofan engines wherein it is desired to suppress turbomachinery-generated noise. In particular, bulk absorbers have been demonstrated to possess a wider suppression band width than typical single degree of freedom systems. Furthermore, the resistance of bulk absorbers tends to be linear with frequency and amplitude, whereas the single degree of freedom systems are nonlinear. However, the adaptation of bulk absorbers to aircraft gas turbine engine applications has been unsuccessful due to their inherent tendency to absorb fluids as well as noise. Fluid contaminants such as water, fuel and solvents are typically present in the gas turbine engine environments to which acoustically treated engine surfaces are exposed. This fluid produces problems such as system weight increase due to fluid absorption, potential structural damage due to fire and freezing and detrimental effects on the acoustic properties of the bulk absorber system.
Previous attempts to adapt bulk absorbers to the gas turbine engine environment have taken two approaches. In certain instances, attempts have been made to treat the relatively porous bulk absorber with a fluid repellant such as a fluorocarbon without significantly changing its acoustic properties. While this attempt has enjoyed limited success in excluding contamination by water absorption, it is unlikely that a universal repellant will be found which will be effective for the wide spectrum of fluids and gases encountered during jet engine operation. In other instances, attempts have been made to fabricate the mechanical equivalent to the acoustic behavior of a bulk absorber. An example of a multiple degree of freedom system exhibiting such behavior may be found in U.S. Pat. No. 3,819,009 - Motsinger, which is assigned to the same assignee as the present invention. However, generally speaking, multiple degree of freedom systems are costly and can only approach the acoustic behavior of a bulk absorber as the number of degrees of freedom, or intersticial partitions, becomes very large. Thus, it becomes desirable to provide an acoustic treatment suppression system which can take advantage of the acoustic properties of typical bulk absorbers while avoiding the problems associated with the contamination of fluids.